1. Field of Invention
The invention relates to a multi-stage circulating fluidized bed (CFB) cooler for cooling a hot gas stream from a reactor while generating both saturated steam and superheated steam. More specifically, the invention is associated with a cooler for cooling the hot syngas from a gasifier handling carbonaceous materials such as coal, biomass or municipal wastes as feed, the cooler simultaneously generates high pressure saturated and superheated steam for power generation. The cooler agglomerates molten ash droplets that are typically present in the syngas generated from a slagging gasifier. The present multi-stage CFB syngas cooler also protects heat transfer surfaces from contacting other fouling, erosive and corrosive substances in the syngas produced by slagging and other types of gasifiers.
2. Background of Art
For those of skill in the art of syngas cooling, difficulties of cooling syngas when directly contacting syngas with the heat transfer surfaces are well-known, and include: plugging the gas flow path due to entrained substances in the syngas, fouling of the heat transfer surfaces due both to deposition of fine molten ash droplets and to tar components in the syngas, erosion due to fine ash and char entrained from the gasifier, and corrosion due to components in the syngas such as hydrogen sulfide and chloride.
Another difficulty associated with syngas cooling is identifying and handling materials of construction for heat transfer surfaces that are compatible with high temperatures and protecting relatively expensive heat transfer surfaces for reliable operation.
At present, options attempting to circumvent the many difficulties of reliably cooling syngas greatly sacrifice process efficiencies. For example, to stay within temperature limits of materials of construction of a conventional convective syngas cooler, the gasifier exit stream must be initially cooled by mixing with large amounts of relatively cooler recycle gas. Other examples of sacrificed process efficiencies in order to accommodate existing syngas coolers include upstream quench cooling, injecting coal in upper portions of the gasifier to lower gasifier exit temperatures, and operating the gasifier at lower temperatures with the attendant lower carbon conversion.
U.S. Pat. No. 8,197,564 discloses an example of quench cooling the syngas downstream of an entrained flow gasifier and radiant syngas cooler to limit downstream plugging and fouling problems normally associated with fine ash and slag that are separated from the gas stream either by precipitating or by surface cooling with direct contact with water. Such quench cooling systems involve an expensive radiant syngas cooler and less-than-reliable water treatment systems to separate particles and treat water as the spent quench water, which is highly corrosive and erosive in nature, increasing overall costs to cool the syngas. In addition, practical experience indicates that the combination of radiant and quench cooling of syngas is not completely effective in limiting (avoiding) plugging problems of a downstream convective cooler.
Syngas generated from fluidized bed gasifiers exits the gasifiers at relatively lower temperatures (approximately 1000° C.) as compared to entrained flow gasifiers. Even then, a syngas cooler to cool the syngas exiting such gasifiers is a relatively expensive piece of equipment due to the use of exotic alloys for the cooling tubes. In a conventional convective cooler that contact the syngas near 1000° C. under high pressure conditions such exotic and expensive alloys must be used. A further difficulty in cooling the syngas from fluidized bed gasifiers is the fine ash and char particles entrained therein that tend to erode the cooling tube surfaces. Deposition and fouling gradually degrades the cooling effectiveness and results in less than desirable superheated steam conditions, affecting generation capacity in an integrated gasification combined cycle (IGCC) plant. To deal with these difficulties and inlet syngas cooler conditions, thick-wall designs comprising exotic alloys have to be used for cooler heat transfer surface materials of construction.
Syngas coolers have limited cooling capacities due to internal hydrodynamics, pressure drop and other process considerations that limit its physical dimensions. In some applications such as in an IGCC process of a nominal 300 MWe capacity, cooling the syngas from a single gasifier requires multiple syngas coolers in parallel. Multiple, parallel syngas coolers in a process line inevitably increase both the costs and layout complexities in handling high pressure syngas near 1000° C.
Processing biomass and bituminous coals in some fluidized bed gasifiers lead to tar formation that entrains with the syngas as it exits the gasifier. The tar components deposit on syngas cooler heat transfer surfaces and downstream equipment and the deteriorating foul conditions eventually lead to an inoperable process. Similar difficulties are encountered while processing coals that contain higher percentage of alkali metals in coal minerals. Even at increased costs and overall decreases in process efficiencies, conventional syngas coolers still cannot be reliably used for these processes with known mitigating measures upstream.
U.S. Pat. No. 4,412,848 discloses a method to cool syngas in a two-stage fluidized bed cooling system. The first-stage fluidized bed cooler operates in the temperature range of 450-500° C. in an attempt to minimize tar condensation on the surface of the inert bed material particles. The second-stage cooler operates in the temperature range of 250-300° C. in an attempt to allow liquid condensation onto the particle surfaces. To avoid solidified condensate accumulation on the surface of the particles, oxygen and steam are injected into the second-stage cooler to burn off the condensate or char on the particle surfaces. This two-stage fluidized bed cooling system advances the art of syngas cooling compared with many other types of heat exchangers for similar applications when the syngas contains condensable liquids or char. It can also generate moderate temperature and high pressure steam to improve the overall process efficiency if the steam is used for power generation. Yet the '848 two-stage fluidized bed cooling system encounters practical difficulties.
One notable disadvantage relates to the substantial amount of oily matter contained in the syngas exiting the cooler that makes it difficult and expensive to treat the sour water that is generated from scrubbing the syngas downstream. Another serious issue is safety—as disclosed, the operating temperature of the second-stage cooler is substantially below the auto ignition temperatures of major components of the syngas such as carbon monoxide (609° C.), hydrogen (500° C.) and methane (580° C.). The operating temperature of 400-500° C. in the combustion zone of the second-stage cooler is lower than the auto ignition temperature of syngas components. Those of skill in the art fully appreciate the danger or increased potential for explosion when injecting oxygen into a syngas stream whose temperature is below the auto ignition temperature. Beyond such safety concerns, the low temperature partial oxidation method necessitates a much larger space for the cooler for a combustion zone and generates much more CO2 than CO.
The cooling capacity of the '848 cooler is also disadvantageous. In a bubbling or spouted bed cooler, the gas superficial velocity is generally below 1 meter per second (m/s). As a result, when large amounts of syngas from a typical IGCC plant needs to be cooled, at least two syngas coolers in parallel are required to avoid the cooler diameter from being above normal transportation limits. Yet parallel cooler arrangements are expensive because the syngas has to be routed to the coolers by refractory lined pipes.
U.S. Pat. No. 5,759,495 discloses a method and apparatus for treating hot gases including syngas in a circulating fluidized bed. It teaches that the gas is sufficiently cooled before it contacts the cooling surface, alleging that erosion of cooling surface in the riser will not be an issue. Yes this teaching oversimplifies a complicated issue. When the cooling surface is in the direct flow path of the riser, where gas superficial velocity is typically above 5 m/s, the erosion of even a cooler cooling surface is inevitable. It is therefore impractical to install the cooling surface inside the riser. Even if not implausible, operating the cooler at such low temperatures generates low grade steam, which is of much less use in a power plant environment. Furthermore, the '495 Patent is silent on how to handle the solids and/or liquid condensate accumulation on the cooler and particle surfaces.
Another internally circulating fluidized bed syngas cooler is disclosed in U.S. Patent Publication No. 2004/0100902. Beneficially, the gas superficial velocity in the disclosed cooler can be operated in the range of 5-10 m/s so that one cooler can handle up to a volume of 90 actual cubic meters per second (m3/s), which relates to a capacity larger than known commercial gasifiers. Although the teaching in this Publication can have wide applications for treating syngas, it too does not disclose how to avoid contaminant accumulations on the particle surfaces and regeneration of bed materials from such contaminants. Furthermore, the Publication discloses a single-stage cooler that does not address the steam conditions necessary for power generation.
To overcome the operability, efficiency and cost issues mentioned above, an improved syngas cooler is highly desirable. It is the intention of the present invention to provide for such an industrial need.